This invention relates to cyanide formation in the lining of electrolytic cells, and more particularly it relates to the suppression of cyanide formation in carbonaceous materials used in electrolytic cells for producing aluminum such as in the carbonaceous linings of Hall cells.
In the Hall-Heroult process for making primary aluminum, aluminum oxide is dissolved in a molten salt such as cryolite and then electrolyzed to form molten aluminum at the cathode. The electrolysis is carried out at a temperature in the range of about 930.degree. to 980.degree. C. The molten salt is contained in a steel shell which is lined with refractories and carbonaceous material. The lining containing the cathode metal, located in the bottom of the cell, is usually made of carbon materials. In addition, refractories are used to maintain thermal conditions in the cell. The amount of carbon used is substantial. For example, a Hall-Heroult cell of moderate size uses about 24,000 pounds of carbon block for lining purposes and uses about 10,000 pounds of carbon ramming paste to complete the lining and to hold the carbon blocks in place. The cell has to be relined about every 4 to 6 years, producing large quantities of used carbonaceous material and refractories, i.e., spent potlining.
Disposing of the spent potlining is not without problems because the lining contains, among other materials, cyanide, e.g., sodium cyanide, typically on the order of about 0.1 wt. %. The amount of cyanide in the used cell liner can vary depending on how long the cell has been used, on the type of carbon used, cell design and how it is operated. The sodium cyanide forms in the liner material during the operation of the cell as a result of sodium, carbon and nitrogen being present. Because the spent potlining contains cyanides, it has been listed by the Environmental Protection Agency as a hazardous waste. Thus, there is a great need for a process that permits the use of the carbonaceous liner without the formation of cyanide.
In the past, numerous approaches have been used to convert the cyanides and to render the spent potliner material safe for disposal. For example, U.S. Pat. No. 5,222,448 discloses that spent potliner is treated by introducing it into a vessel, and exposing it to the heat of a plasma torch at a temperature of at least 1000.degree. C. As a result, carbon is gasified and convened to combustible carbon monoxide or hydrocarbons, or to carbon dioxide; inorganic material is melted to form slag; fluoride compounds are melted, vaporized, or reduced to gaseous HF; cyanide compounds are destroyed; and all other materials, including sulfur compounds, are either melted or gasified. As a result, the spent potliner is rendered non-hazardous, and the quantity of remaining slag has both its solid volume and mass substantially reduced by a factor of at least 1.5:1 in mass and at least 3:1 in volume relative to the input spent potliner.
U.S. Pat. No. 4,576,651 discloses a process for treating fluoride-contaminated scrap lining material from electrolytic reduction cells which comprises mixing the material with 7-30 parts of sulfuric acid and sufficient water to bring liquid content to 60-80 parts per 100 parts of lining material, mixing in sufficient lime to at least neutralize the sulfuric acid and make the slurry slightly alkaline, the slurry then being allowed to set into a solid mass. The slurry should be of a paste-like consistency. The lime may be wholly calcium hydroxide, but a substantial proportion may be in the form of calcium carbonate. The scrap, before or after the above treatment with lime and sulfuric acid, is preferably heated to 150.degree.-500.degree. C. in the presence of water vapor to destroy cyanides.
U.S. Pat. No. 4,763,585 discloses a process for the combustion of ground, spent potlinings generated during the production of metallic aluminum. The process includes grinding the potlinings to a particle size of not greater than about 2 inches in any dimension; mixing with the ground potlinings from about 1 to about 20 wt. %, based upon the weight of the potlinings, of a powdered inert additive having a median particle size of not greater than 10 micrometers, and burning the ground potlinings in a combustor at a temperature in the range of from 1400.degree. F. to about 2200.degree. F., the additive coating the ground potlinings and preventing their agglomeration in the combustion zone therein.
U.S. Pat. No. 4,973,464 discloses a method for removal of cyanides from spent potlinings from aluminum manufacture. The method discloses the treatment of ground, spent potlinings generated during the production of metallic aluminum to reduce cyanide content to environmentally nonhazardous levels. Potlinings are ground or otherwise suitably reduced in size to a particle size of not greater than about 2 inches in any dimension and roasted in a stream of air or nitrogen at a temperature between about 500.degree. and 1400.degree. F. Roasting for an appropriate time-temperature interval reduces cyanide content to desired levels without combustion of a major portion of carbonaceous material, resulting in an end product rich in carbon and fluorine which may be salable because of this content.
U.S. Pat. No. 4,993,323 discloses that an environmentally acceptable and effective method for thermal destruction of Spent Potliner (SPL) by Fluidized Bed Combustion (FBC) has been established. This method has overcome problems associated with ash agglomeration, ash leachate character and emission control, the primary obstacles for applying FBC to the disposal of SPL and like solid fuels. Specifically, "recipes" of appropriate additives (fuel blends) are proposed. A mixture of lignite, limestone and SPL in an appropriate proportion has proven to notably increase the agglomeration temperature of the ash, allowing this low-melting waste to be destroyed continuously by FBC. Ash leachate character is modified by control of ash chemistry to ensure that fluoride anions and metallic cations are at or below acceptable limits.
U.S. Pat. No. 5,024,822 discloses a process for treating spent potlining from the electrolytic smelting of aluminum in cryolite including incinerating the potlining to combust carbonaceous material to forth an ash at a temperature low enough to maintain low fluorine vapor pressures, admixing siliceous material with the potlining either before or after the ash-forming stage, and heating the ash and siliceous material to form a glassy residue.
In Norwegian Disclosure 175,159, the cyanide-containing potlining is treated in situ by raising the cell temperature before shut-down of the cell, thus promoting penetration of electrolyte into the lining to react with the cyanide.
However, it will be noted that these treatments are post-treatments to correct the hazardous waste problems resulting from spent potlinings, and most of them are relatively expensive. Thus, it will be seen that there is a great need for a method that permits the use of carbonaceous liners but is effective in preventing formation of undesirable compounds such as cyanide compounds during use of the cell to produce aluminum. By preventing formation of compounds such as cyanide compounds, any post-treatment can be greatly simplified.